The present invention relates to structure for providing balanced pressure to a gear pump during reduced flow operation.
Gear pumps are well known and utilized in many industrial applications. When used in hydraulic systems gear pumps may be operable to move fluid from a sump to a high pressure user system. These gear pumps may be constantly driven. When it is not desired to have the pump move fluid, some means of reducing the fluid moved by the constantly rotating gears is used. A valve typically restricts the inlet flow to the pump.
A known gear pump system is illustrated in FIG. 1. Gear pump 20 consists of a pair of gears 22 and 24 mounted within casing 26. Internal surface 28 is formed within casing 26 and defines a pump chamber to receive gears 22 and 24.
Sump 29 supplied fluid through inlet 30 into the pump chamber, and gears 22 and 24 rotate within the chamber to move fluid around their outer peripheries to outlet 32. Outlet 32 delivers the fluid through a pressure gage 34 and to a user system 36. A high pressure typically exists at outlet 32.
A force is applied to gears 22 and 24 from the high pressure fluid on the discharge side in a direction towards the inlet side. This force F directs the gears against the internal surface 28 of casing 26 in a direction generally perpendicular to the rotational axes of the gears and towards inlet 30. Due to force F, gears 22 and 24 contact internal surface 28 and material is removed from internal surface 28 until groove 38 is formed. Groove 38 is customized for the particular gears 22 and 24 and casing 26. The removal of material, or "tracking in" occurs during initial use of the gear pump and ensures a close fit between the tips of gear teeth 35 and internal surface 28. Internal surface 28 is quite hard, and as gears 22 and 24 remove material to form groove 38, the tips of gear teeth 35 may also be removed.
Contact line 33 is shown for gear 24. The spaces between adjacent gear teeth 35 past contact line 33, and towards outlet 32, contain high pressure fluid. Thus, there is high pressure fluid at positions between a center line 39 of casing 26 and contact line 33. Center line 39 could be defined as the intersection of a plane defined by the axes of gears 22 and 24, and internal surface 28. The high pressure fluid in the space between contact line 33 and center line 39 associated with gear 24 applies a force in a direction upwardly and to the right, as shown in FIG. 1. This force balances a force on the opposed side of gear 24 which is forcing it downwardly and to the left as shown in FIG. 1. Thus, the resultant force F on gear 24 is directly to the left as shown in FIG. 1, or in a direction towards inlet 30. Mirrored forces are applied to gear 22.
Inlet valve 40 is mounted on inlet 30 and can be actuated to restrict the flow of fluid from sump 29 into pump chamber 28. This would occur when it is not desired to have fluid delivered to system 36, but it is still desired to supply a small amount of fluid for bearing lubrication to rotating gears 22 and 24. This is known as "dry valve" operation. In such cases valve 40 is moved to the position illustrated in FIG. 2 and the flow into pump chamber 28 is restricted. At these low flow conditions a high vacuum is placed on inlet 30 which removes dissolved air from the fluid in the system. Air bubbles fill the spaces between adjacent gear teeth.
As shown in FIG. 2, the inter tooth space between center line 39 and contact line 33 now contains air rather than high pressure fluid. The air bubbles continue to rotate towards outlet 32 until they contact high pressure fluid, at which time they collapse. There is still high pressure fluid adjacent outlet 32, forcing gear 24 downwardly and to the left, but there is no longer high pressure fluid directing a force upwardly and to the right as shown in this figure. Thus the resultant force F is now downwardly and slightly to the left from the rotational axis of gear 24 and upwardly and slightly to the left from the rotational axis of gear 22. Gears 22 and 24 now move in these directions and new tracking grooves 42 are formed. The tips of gear teeth 35 experience additional wear tracking in groove 42.
When the pump returns to normal operation, there is no longer contact between gear teeth 35 and the casing at positions near contact line 33. The gear teeth tips have been removed such that there is undesirable clearance between gear teeth 35 and bore 28 near contact line 33, and perhaps throughout the entire circumferential extent of internal surface 28. This causes undesirable leaking.
Operating the gear pump under conditions such as extremely high vehicle attitude or low fluid levels could also result in the above-described problem. These conditions could result in a temporary uncovering of the inlet line in the fluid reservoir. When this occurs, large volumes of air could be introduced into the inlet causing a problem similar to the above-discussed problem.
Another problem that occurs when air is in the spaces between gear teeth is that pressure balanced side plates may be forced into the gears, such that the side plates could be torn or smeared. The side plates are typically forced against the gears by discharge pressure on a side remote of the pump chamber. This force is balanced by the pressure from the pump fluid within the pump chamber. In the absence of such pressure the side plates may be forced against the gear by an unbalanced force which could damage the side plates.